Brightening and cleaning composition and treatment for magnesium and magnesium-base alloys



BRIGHTENING AND CLEANING COMPOSITION AND TREATMENT FOR MAGNESIUM ANDMAGNESIUM-BASE ALLOYS Lawrence Whitby, Midland, Mich., assignor to TheDow Chemical Company, Midland, Mich., a corporation of Delaware NoDrawing. Filed Nov. 3, 1958, Ser. No. 771,680

6 Claims. (Cl. 156-20) The invention is related both to surfacefinishing of magnesium and magnesium-base alloy articles and to theremoval of scored-in layers resulting from the lubricant used inhot-forming shapes of magnesium metal and its alloys. Magnesium alloysare considered herein to be magnesium-base alloys containing at least 85weight percent of magnesium.

Magnesium and magnesium-base articles, e.g., shapes produced byextruding, drawing, stamping, rolling, diecasting, sand-casting, andpermanent mold casting, and articles fabricated therefrom have a dullappearance due largely to tarnishing and scoring of the surface duringthe shaping operation, and/or attack of the metal surface by ambientgases. Such articles and shapes are customarily given a bright finishtreatment to impart greater protection and attractiveness thereto. Suchbright finishes are usually obtained by contacting the metal articlewith an acid solution, e.g., nitric acid or mixtures of aqueous chromicacid and sodium nitrate. The article may be immersed in the acidsolution or the solution may be sprayed or brushed on the article. Knowncompositions and methods of providing a bright finish on magnesium andmagnesium alloy articles have a number of drawbacks which include (1)excessive metal removal, (2) rapid depletion of the bath made from suchcompositions, (3) ineificacy of the composition on certain die-castalloys, e.g., those designated AZ91 by the American Society for TestingMaterials, and (4) staining and discoloration of articles which containsmall cracks when treated with chromic acidcontaining solutions as aresult of which the colored liquid migrates into the cracks from whichit later bleeds out over a prolonged period of time following thetreatment.

Graphite is commonly employed as one of the ingredients in lubricantsused in hot-forming operations. A serious problem has accompanied theemployment of such lubricants for shaping magnesium and its alloys. Thegraphite lubricant is burned in or scored into the contacting surfacethereof to produce layers which are extremely difficult to remove. Suchlayers or even residual traces thereof on the metal surface mayinterfere with the metal finishing, particularly electroplating, andstimulate wet corrosion on the magnesium or alloy by the electrolyticaction between the graphite serving as the cathode and the magnesiumserving as the anode.

Heretofore, the most widely used method of removing burnt and scored-ingraphite lubricant surface imperfections has been a two-stage treatmentconsisting first of a 10 to 20 minute soaking period in a near-boilingcaustic soda solution and then an immersion in an acid pickle bathconsisting essentially of an aqueous mixture of chromic acid and sodiumnitrate whereby at least 0.5 mil of metal from the surface was removed.The disadvantages of the known methods of removing this scored-inmaterial include those set out hereinabove, particularly a prolongedtreatment time, an undesirably high metal removal, a lack ofeffectiveness of the known solutions to loosen and dislodge the deeplyscored-in graphite, and the cost of such known methods due to thenecessity of frequent replacement of active ingredients in the exhaustedsolution.

United States Patent 3,004,879 Patented Oct. 17, 1961 The problemsabove-mentioned which are associated with the brightening of magnesiumand magnesium alloy articles and to the necessity of removal ofscored-in graphite lubricant and the accompanying imperfections on thesurface of shaped magnesium and magnesium-base alloy parts have createda long-existing desideratum in the art for an improved composition andmethod of use thereof for removing such graphitic lubricant residues andfor brightening magnesium shapes and fabricated articles generally.

It is, therefore, the principal object of the invention to satisfy thisdesideratum by providing an improved composition and method of usethereof for the removal of graphitic lubricant from hot-formed magnesiumand magnesium alloy shapes and to brighten such shapes and fabricatedarticles made of such shapes by employing a composition of an alkylhydrogen ortho-phosphate and nitric acid.

The method of the invention consists essentially of contacting themagnesium or magnesium-base alloy article requiring brightening, or theremoval of graphite lubricant therefrom, with the composition of theinvention by immersing the article therein, or otherwise applying thesolution to the magnesium article, for a period of time of between 0.5and 15 minutes, removing the article from contact with the solution, andrinsing adhering solution therefrom with water. A high degree ofbrightening occurs but only a small amount of metal is lost because anyappreciable loss is prevented by the formation of an inhibiting film onthe surface thereof while being subjected to the action of the solutionaccording to the invention.

The alkyl hydrogen ortho-phosphate employed according to the inventionis either the mono-hydrogen or the di-hydrogen ester or mixtures thereofof an alkyl alcohol of five carbon atoms or less. The esters are thoseformed by reacting phosphoric acid and a lower alkyl alcohol. Themonohydrogen ester may be expressed generically as:

OH ROl =O the dihydrogen ester as:

(3H HOP=O where R is an alkyl group of 1 to 5 carbon atoms.

The ratio of the ortho-phosphate ester to nitric acid to use inpreparing the composition of the invention is between 10 and 100 volumesof a 52 to percent by weight aqueous solution of HNO; to volumes of theester. Lower concentrations add more water than is desirable which bothincreases the dissolution activity of the mixtures for magnesium alloysand decreases the luster obtainable when more concentrated nitric acidis used.

A preferable concentration of nitric acid for admixture with the alkylhydrogen ortho-phosphate ester is between a 60 and 90 percent by weightaqueous solution of HNO and the recommended concentration of nitric acidto use is between a 65 and 75 percent by weight aqueous solution.

A proportion of less than 10 volumes of aqueous nitric acid to 100volumes of the ester will brighten oxide-free magnesium and magnesiumalloys but will dissolve oxides only very sluggishly and thence prolongthe treatment unduly if metal having an oxidic tarnish is to be treatedwithout prior cleaning. There is also a larger loss due to drag-out,i.e., adhering treating solution when the article being treated isremoved from the solution, if the num, magnesium-base alloy article.

proportion of nitric acid in the aqueous acid-ester mixture is too low,e.g., under percent by volume, due to the increased viscosity.

The temperature of the bath when applied should be not over 30' C. andpreferably should be between C. and 22 C. At a temperature higher than30 C., the rate of dissolution of the metal tends to rise to undesirableamounts and much of the luster of the finish obtained at the lowertemperatures is lost.

A recommended ratio by volume of phosphoric acid ester to nitric acidfor use with magnesium-base alloys may be broadly stated to be 100 ofthe phosphoric acid ester to about of the nitric acid, but for alloyscontaining 6 percent or more of aluminum which includes a large numberof cast alloys, a preferred volume ratio of the ester to the acid is 100of the ester to between 50 and 80 of the acid. By increasing theproportion of nitric acid, the rate of dissolution of such alloys in themixture is decreased and the resulting luster is concurrently increased.The increased proportion of nitric acid also lessens the cost of thetreating composition. The length of time usually used for immersing anarticle composed substantially of magnesium or of a low aluminummagnesium-base alloy, i.e., one containing less than about 6 percent Al,into the composition of the. invention, or otherwise applying thecomposition to the magnesium article as by spraying, is between 0.5 and2 minutes dependent upon the amount of scale to be removed. Usuallybetween 0.5 and 1 minute is ample time for providing a bright finishupon such a low alumi- However, a period up to 15 minutes is sometimesemployed to brighten high aluminummagnesium-base alloys and to removethe graphite lubricant which is scored into the sides of drawn orextruded articles of such alloy.

Magnesium or magnesium-base articles thickly coated with graphiticlubricant often appear little affected by the composition of theinvention for up to an initial period of as long as two minutes oroccasionally a little longer but after such initial period, thegraphitic layer abruptly loosens and falls away.

The practice of the invention will be better understood by anexamination of the following examples which are illustrative of theinvention.

Example 1 A bath of the composition of the invention was made up byplacing 100 volumes of technical grade ethyl hydrogen ortho-phosphate,consisting of approximately equal parts by volume of the monoanddi-hydrogen phosphoric acid esters, in a stainless steel container. Thecontainer was provided with an exhaust system to remove any offensivegases, an agitator for maintaining substantially uniform temperature andcomposition, and a cooling system for temperature control. 55 volumes ofconcentrated nitric acid, having a specific gravity of 1.40 andconsisting of a 67 percent by weight aqueous solution of HNO were addedwith stirring into the phosphoric acid ester. The mixture wasyellowish-brown in color and evolved small quantities of nitrogendioxide, but, upon aging for several days, the mixture became almostcolorless and could be heated to at least 80 C. without noticeablereaction. A die casting of a magnesium alloy consisting essentially of 9percent Al, 1 percent Zn, about 0.2 percent Mn, and the balance Mg,having a tarnished exterior, was immersed in the bath of the inventionat 24 C., and agitated therein for 30 seconds. It was thereafterremoved, drained from 15 to 30 seconds and thoroughly rinsed with water.After drying, the magnesium casting showed a highly bright finish. Theweight loss showed that such finish was obtained with less than 0.1 milof metal removal on each surface.

4 Example 2 A bath of the invention was prepared by admixing 100 volumesof technical ethyl hydrogen ortho-phosphate and 25 volumes of the 67percent by weight aqueous solution of HNO (specific gravity of 1.40) ina stainless steel container similarly as in Example 1. The bath thenstood for 24 hours after which a graphite-coated deep-drawn, hot-formedmagnesium alloy shape, consisting essentially of 3 percent Al, 1 percentZn, about 0.2 percent Mn, and the balance Mg was immersed in the bath at22 C. and retained therein for 1.5 minutes. At the end of that time, thegraphite contaminant adhering to the surface peeled away suddenlyleaving only the most deeply scored imperfections. After an immersion ofan additional 3 minutes, the surface of the metal was completely free ofany graphite blemishes; the shape was then removed from the bath,drained for 15 seconds, and rinsed in running water at room temperature.The graphite contaminant removed by the above means resulted in a brightclean metal surface of a high luster. The total metal removed from themagnesium piece was only 0.1 mil per side or a total reduction in sizeof the article between parallel faces of 0.2 mil.

Example 3,

A bright finish bath in accordance with the invention was prepared as inthe above examples except that volumes of the 67 percent by weightaqueous solution of HNO were admixed with volumes of technical gradeethyl hydrogen ortho-phosphate. A metal casting consisting ofsubstantially 9 percent Al, 1 percent Zn, about 0.2 percent Mn, and thebalance Mg was immersed in the bath thus prepared and allowed to remaintherein for 2 minutes at a temperature of 21 C. and thereafter removedand rinsed in running water at tap water temperature. A high luster wasthus obtained with a metal removal of less than 0.1 mil per side.

Example 4 Composition in Time of Loss of Alloy percent immersion metalin Appearance in minutes mils side M alloy. 1.2 Mn, balance Mg- 1 0.1Bright luster. 111131---. 3%;IIh, 6 Zr, balance 1 0. 1 Do.

g. ZK60-... (Sign, 0.5 Zr, bal- 1 0.1 Do.

The phosphoric acid esters most commonly employed in preparingcomposition of the invention are the diand mono-hydrogenortho-phosphates of methyl, ethyl, and isopropyl. Phosphoric acid estersof other alcohols, e.g., propyl, butyl, and amyl, may be used, butarticles treated in such compositions are more difiicult to rinse.

A number of outstanding advantages stem from the practice of theinvention. A high luster is obtained on cast, drawn, extruded orotherwise shaped articles of magnesium and magnesium-base alloys whichis superior to that obtained heretofore by any known bright pickle orfinish composition or method of providing a finish on such articles. Theinvention is particularly adapted for the efi'icient removal of scoredsurface layers due to graphite from the lubricant employed in theshaping thereof. The degree of etch and amount of metal lost areextremely low. Usually the scored surface layer drops away from themagnesium article in the form of a thin peel within a short time afterthe immersion of the scored piece in the composition in accordance withthe practice of the invention. The advantages of the invention havefar-reaching economic significance. The long and tedious method ofbrightening the surface of magnesium articles resulting in substantialmetal loss according to known practice may now be replaced by the fastacting and more efiicient method of the invention, which employsrelatively low-cost and readily available ingredients, to produce abrighter finish with a relatively low metal loss. In addition, thecomposition of the invention is an excellent solvent and/or degradingagent for oils and greases and the invention may, therefore, be used onmetal surfaces having oily or greasy coatings thereover without priordegreasing treatment.

Having described the invention, what is claimed and desired to beprotected by Letters Patent is:

1. The composition for cleaning and brightening the surfaces of articlesof magnesium and magnesium-base alloys which consists by volume of 100parts of an alkyl hydrogen ortho-phosphate having up to 5 carbon atomsin the alkyl group and between 10 and 100 parts of an aqueous solutionof nitric acid containing between 52 and 95 percent by weight of HN 2.The composition for cleaning and brightening the surfaces of articles ofmagnesium and magnesium-base alloys which consists by volume of 100parts of a mixture of alkyl mono-hydrogen ortho-phosphates and alkyldihydrcgen orthophosphates having up to 5 carbon atoms in the alkylgroup and between 20 and 80 parts of an aqueous solution of nitric acidcontaining between 60 and 90 percent by weight of HNO 3. A compositionfor cleaning and brightening the surfaces of articles of amagnesium-base alloy which consists by volume of 100 parts of an alkylhydrogen orthophosphate ester selected from the class consisting of themono-hydrogen and the dihydrogen esters of methyl, ethyl, isopropyl, andbutyl phosphoric acid esters and mixtures thereof, and between 50 and 80parts of an aqueous solution of nitric acid containing between 65 and 75percent by weight of HNO 4. The method of cleaning and brightening thesurfaces of a metal article composed of at least 85 percent magnesiumconsisting of admixing by volume 100 parts of an alkyl hydrogenortho-phosphate with between and 100 parts of an aqueous solution ofnitric acid containing between 52 and 95 percent by weight of HNO andcontacting the surfaces of said article with the solution so made, at atemperature of between 15 and 30 C. for between 0.5 and 15 minutes.

5. The method of cleaning and brightening the surfaces of a metalarticle containing at least 85 percent magnesium consisting of admixing100 parts by volume of a mixture of alkyl mono-hydrogen ortho-phosphatesand alkyl dihydrogen ortho-phosphates having up to 5 carbon atoms in thealkyl group and between 20 and 80 parts of an aqueous solution of nitricacid containing between and 90 percent by weight of HNO contacting thesurfaces of said article with the solution so made, at a temperature ofbetween 15 and 22 C. for between 0.5 and 15 minutes, removing thearticle from contact therewith, and rinsing with water.

6. The method of removing graphite lubricant adhering to the surfaces ofa magnesium-base alloy article containing at least 85 percent Mg and atleast 6 percent Al, consisting of admixing by volume 100 parts of amixture of esters consisting of ethyl mono-hydrogen and ethyl dihydrogenortho-phosphates and between 50 and 80 parts of an aqueous solution ofnitric acid containing between and percent by weight of HNO immersingsaid article in the solution so made, at a temperature of between 15 and22 C. for between 0.5 and 15 minutes, removing the article therefrom,and rinsing with water.

References Cited in the file of this patent UNITED STATES PATENTS1,740,731 Gravell Dec. 24, 1929 1,949,713 Gravell Mar. 6, 1934 2,197,405Edwards Apr. 16, 1940 2,408,155 Thornbury Sept. 24, 1946 2,446,060 Prayet al. July 27, 1948 2,553,937 Patric May 22, 1951 2,729,551 Cohn Jan.3, 1956 FOREIGN PATENTS 519,218 Canada Dec. 6, 1955 740,879 GreatBritain Nov. 23, 1955 OTHER REFERENCES Trend in Alum. Clug Harris, April1945, in Alum. and Mg, publ. Pages 2832. Table 1 on page 29.

4. THE METHOD OF CLEANING AND BRIGHTENING THE SURFACES OF A METALARTICLE COMPOSED OF AT LEAST 85 PERCENT MAGNESIUM CONSISTING OF ADMIXINGBY VOLUME 100 PARTS OF AN ALKYL HYDROGEN ORTHO-PHOSPHATE WITH BETWEEN 10AND 100 PARTS OF AN AQUEOUS SOLUTION OF NITRIC ACID CONTAINING BETWEEN52 AND 95 PERCENT BY WEIGHT OF HNO3 AND CONTACTING THE SURFACES OF SAIDARTICLES WITH THE SOLUTION SO MADE, AT A TEMPERATURE OF BETWEEN 15* AND30* C. FOR BETWEEN 0.5 AND 15 MINUTES.